A. Field of the Invention
This invention relates generally to the field of orthodontics. More particularly, the invention relates to methods for designing and manufacturing brackets and archwires for purposes of straightening the teeth of a patient, and novel brackets and archwires made in accordance with the methods. The invention is useful for orthodontics generally. It can be employed with particular advantage in lingual orthodontics, that is, where the orthodontic appliance is attached to the lingual surface of the teeth for aesthetic reasons.
B. Description of Related Art
A widely used method to straighten or align teeth of a patient is to bond brackets onto the teeth and run elastic wires of rectangular cross-sectional shape through the bracket slots. Typically, the brackets are off-the-shelf products. In most cases, they are adapted to a certain tooth (for instance an upper canine), but not to the individual tooth of a specific patient. The adaptation of the bracket to the individual tooth is performed by filling the gap between tooth surface and bracket surface with adhesive to thereby bond the bracket to the tooth such that the bracket slot, when the teeth are moved to a finish position, lies in flat horizontal plane. The driving force for moving the teeth to the desired finish position is provided by the archwire. For lingual brackets, a system has been developed by Thomas Creekmore that has vertical bracket slots. This allows an easier insertion of the wire. The longer side of the wire is therefore oriented vertically. Unitek has marketed this bracket system under the trade name CONSEAL(trademark).
A computerized approach to orthodontics based on design and manufacture of customized brackets for an individual patient, and design and manufacture of a customized bracket placement jig and archwire, has been proposed in the art. See U.S. Pat. No. RE 35,169 to Lemchen et al. and U.S. patents to Andreiko et al., U.S. Pat. Nos. 5,447,432, 5,431,562 and 5,454,717. The system and method of Andreiko et al. is based on mathematical calculations of tooth finish position and desired ideal archform. The method of Andreiko et al. has not been widely adopted, and in fact has had little impact on the treatment of orthodontic patients since it was first proposed in the early 1990s. There are a variety of reasons for this, one of which is that the deterministic approach proposed by Andreiko et al. for calculating tooth finish positions does not take into account unpredictable events during the course of treatment. Furthermore, the proposed methods of Andreiko et al. essentially remove the orthodontist from the picture in terms of treatment planning, and attempt to replace his or her skill and judgment in determining tooth finish positions by empirical calculations of tooth finish positions.
Typically, the wires used in orthodontic treatment today are off-the-shelf products. If they need to be individualized by the orthodontist, the goal is to get along with as few modifications as possible. Therefore, the brackets are designed in a manner that at the end of treatment, when teeth are aligned, the bracket slots are supposed to be located and oriented in a planar manner. This means that a wire that would run passively through the slots, without applying any force, would be planar (flat). This treatment regime is known as xe2x80x9cstraight wirexe2x80x9d. It dominates orthodontics worldwide. It is efficient for both manufacturers and the orthodontist. The customized orthodontic appliances proposed by Andreiko et al. call for a flat planar wire, but with the curvature in a horizontal plane customized for the individual and dictated by the shape of the ideal desired archform for the patient.
The so-called straight wire approach that continues to be used in orthodontics today has some noteworthy disadvantages in terms of patient comfort. The need to close the gap between the bracket bonding surface and the tooth surface with adhesive always leads to an increased overall thickness of the appliance. For brackets that are bonded labially, this is acceptable, as labial tooth surfaces are very uniform for different individuals, and the gap to be closed is not significant. However, lingual (inner) surfaces of teeth show a much greater variation among patients. To achieve the goal to orient the bracket in a manner such that the slot is parallel to all other slots when treatment is finished, the thickness of adhesive that is necessary often is in the range of 1 to 2 mm. It is obvious that every fraction of a mm added to appliance thickness significantly increases patient discomfort. Especially with lingual brackets (bracket bonded to the lingual surface of the teeth), articulation problems arise, and the tongue is severely irritated for several weeks after bonding. The tooth surfaces next to these adhesive pads are difficult to clean, thus serving as collecting point for bacteria and causing gingival inflammation. The further the archwire is away from the tooth surface, the more difficult it is to achieve a precise finishing position for each tooth. An error of only 10xc2x0 in torque (rotation around the wire axis) may well induce a vertical error in tooth position of more than 1 mm.
Another significant disadvantage of thick brackets, especially when bonding lingually, arises when the front teeth are severely crowded (which is often the cause for orthodontic treatment). Since the space is more restricted at the lingual surface due to the curvature of the jaw, not all brackets may be bonded at one session. Rather, the orthodontist has to wait until the crowding has decreased until all brackets may be placed. Crowding also creates problems for labial brackets. Geometrical considerations dictate that this constriction problem becomes worse as the thickness of the bracket/bracket bonding pad/adhesive combination increases.
Another problem in orthodontics is to determine the correct bracket position. At the time of bonding, teeth may be oriented far away from the desired position. So the task to locate the brackets in a manner that a flat planar archwire drives teeth to the correct position requires a lot of experience and visual imagination. The result is that at the end of treatment a lot of time is lost to perform necessary adjustments to either bracket position or wire shape. This problem can be solved by creating an ideal set-up, either virtually using 3D scan data of the dentition or physically by separating a dental model of the dentition into single teeth and setting up the teeth in a wax bed in an ideal position. The brackets can then be placed at this ideal set-up at optimal positions, in a manner that a flat wire running through the bracket slots would drive the teeth exactly into the ideal target. This again may be done virtually in a computer or physically. After this is done, the bracket position has to be transferred on a tooth-by-tooth basis into the maloccluded (initial) situation. Basing on this maloccluded situation, a transfer tray enveloping the brackets can be manufactured, which allows bonding the brackets exactly at the location as defined at the set-up. Such as technique is taught generally in Cohen, U.S. Pat. No. 3,738,005.
The published PCT patent application of OraMetrix, Inc., publication no. WO 01/80761, describes a wire-based approach to orthodontics based on generic brackets and a customized orthodontic archwire. The archwire can have complex twists and bends, and as such is not necessarily a flat planar wire. The entire contents of this document is incorporated by reference herein. This document also describes a scanning system for creating 3D virtual models of a dentition and an interactive, computerized treatment planning system based on the models of the scanned dentition. As part of the treatment planning, virtual brackets are placed on virtual teeth and the teeth moved to a desired position by a human operator exercising clinical judgment. The 3D virtual model of the dentition plus brackets in a malocclused condition is exported to a rapid prototyping device for manufacture of physical model of the dentition plus brackets. A bracket placement tray is molded over the model. Real brackets are placed into the transfer tray in the location of where the virtual brackets were placed. Indirect bonding of the brackets to the teeth occurs via the transfer tray. The system of WO 01/80761 overcomes many of the problems inherent in the Andreiko et al. method.
During the course of treatment, brackets may come off, for instance if the patient bites on hard pieces of food. Obviously, the transfer tray used for initial bonding will not fit any more as teeth have moved. While it is possible to cut the tray (such as described in WO 01/80761) into pieces and use just the one section that is assigned to the bracket that came off, to replace the bracket the reliability of this procedure is limited, as a small piece of elastic material is not adequate to securely position a bracket. It may therefore be required to create a new transfer tray adapted to the current tooth position using a costly lab process.
The methods and applicants presented herein comprise several independent inventive features providing substantial improvements to the prior art. The greatest benefits will be achieved for lingual treatments, but labial treatments will also benefit. While the following summary describes some of the highlights of the invention, the true scope of the invention is reflected in the appended claims.
In a first aspect, a set of brackets (one or more) is provided in which the bracket has a slot which is oriented with respect to the bracket bonding pad such that the wire runs substantially parallel to the surface of the teeth, i.e., the portion of the tooth surface adjacent to where the bracket receives the archwire, as will be explained in further detail and as shown in the drawings.
In particular, the brackets have a bracket bonding pad for bonding the bracket to the tooth of the patient and a bracket body having a slot for receiving an archwire having either a flat, planar side (e.g., one side of a wire having a rectangular, square, parallelogram or wedge-shaped cross-sectional shape) or alternatively an oval shape. The slots of the brackets are oriented in approximate parallel alignment relative to its respective bracket bonding pad in a manner such that, when the bracket or set of brackets are installed on the teeth of the patient and the archwire is inserted in the slots, the archwire is canted or inclined relative to the occlusal plane (analogous to a banked curve on a high speed racing track). In embodiment in which the archwire has flat surfaces (rectangular, parallelogram, square, wedge shaped, etc), the flat planar side of the archwire is substantially parallel to the surface of the teeth at the location of where the archwire is inserted into the slots, in a canted orientation relative to the occlusal plane. In an embodiment in which the archwire is of an oval configuration, the major axis of the cross-section of the wire is oriented substantially parallel to tooth surface and at a canted orientation relative to the occlusal plane.
For the front teeth, it is desirable to come up with a homogeneous inclination to avoid abrupt changes in inclination (i.e., changes in torque) from slot to slot in order to receive a smooth progression of the wire. In a wire of rectangular or square cross-sectional shape, one of the pairs of parallel opposite sides of the archwire is oriented substantially parallel to the tooth surface. Usually, this will be pair of parallel sides that has the greater width or height. This aspect of the invention enables the overall thickness of brackets to be substantially decreased as compared to prior art techniques, because it does not require a buildup of adhesive to make the slot lie in a horizontal flat plane when the bracket is attached, as found in the straight wire technique. The brackets and archwire design are particularly well suited for use in lingual orthodontics.
This reduction in thickness of the bracket, bracket bonding pad and archwire leads to several significant advantages as compared to prior art systems and satisfaction of a long-felt need in the art for a more satisfactory lingual orthodontic system. These advantages include decreased articulation problems, a pronounced decrease in tongue irritation, a decreased risk of bracket loss, increased positioning control for finishing since the reduced distance between wire and tooth results in more accurate tooth movement to the desired finish position, increased patient comfort, and increased hygiene conditions.
One reason why the basic design of orthodontic wires remains one in which the wires have a flat, planar shape is the ease of industrial manufacturing. To decrease the thickness of an orthodontic bracket, it is much preferable to run the wire parallel to the surface of each individual tooth as provided by this aspect of the invention. The lingual surfaces of front teeth are significantly inclined relative to a vertical axis for most patients. A wire that runs parallel from tooth to tooth in accordance with this aspect of the invention has a xe2x80x9ccantedxe2x80x9d shape in order to take advantage of the parallel nature of the bracket slots. Using standard mass-production procedures, such a wire could not be fabricated, as every patient has a very individual tooth anatomy. Shaping a wire manually to provide the canted shape is extremely challenging. Usage of modem materials for the archwire like shape memory alloys makes this task even more challenging or even impossible by hand. However, in a preferred embodiment of the present invention the required wire geometry is available in electronic format. This wire geometry can be dictated by the three-dimensional location of the bracket slots and/or the brackets, as placed on the teeth in the desired occlusion. This format can be exported to new wire bending robots that have been recently developed that are capable of bending wires in virtually any shape (including canted shapes). For example, it is possible to export digital data reflecting wire geometry to flexible wire bending production devices like the 6-axis-robot described in WO 01/80761, and have the robot bend and twist wires of the canted configuration as described herein. Thus, wires having the canted shape as dictated by the bracket invention are now able to be mass-produced. The presently preferred wire-bending robot is also described in U.S. patent application Ser. No. 09/834,967, filed Apr. 13, 2001, the content of which is also incorporated by reference herein in its entirety.
Thus, in another and related aspect of the invention, a canted archwire is provided. The wire can be of any cross-sectional configuration that has at least one flat planar surface, such as rectangular, or, alternatively, it could be oval in cross-section. The archwire is bent into a configuration during manufacturing to have a shape, in a relaxed, as-manufactured condition, such that the flat planar surface of the archwire (or the major axis of the cross-section of the wire in an oval configuration) is canted relative to an occlusal plane over a substantial arcuate extent. The canting of the archwire corresponds to portions of the archwire that are to be placed in brackets and used for straightening two or more teeth. In an embodiment in which the wire is of rectangular or square cross-section, one of the first and second pairs of parallel sides is oriented substantially parallel to tooth surfaces in the vicinity of where the archwire is to be received by archwire receiving receptacles located on the two or more teeth.
Another aspect of the invention is thus a method of manufacturing an archwire. The method includes the step of defining the location of a set of bracket slots for a set of brackets in three-dimensional space with the aid of a computer. The bracket slots are oriented substantially parallel to the surface of the teeth in the location of where the brackets are to be bonded to the teeth. The method continues with the step of supplying a wire bending robot with information corresponding to the location of the set of bracket slots. This information will be typically in the form of a digital file representing 3D coordinates of the bracket slots. This information can be used by a robot control program to tell a wire bending robot how to bend a wire such that the wire, in a relaxed, as manufactured state, has a shape dictated by the bracket slots. Thus, the method continues with the step of bending an archwire with the wire bending robot having a shape corresponding to the location of the bracket slots, wherein the archwire has a canted configuration such that the archwire is oriented substantially parallel to the tooth surfaces over a substantial arcuate extent. The wire can be bent continuously, or, alternatively, as series of bends separated by straight section corresponding to the bracket slots, as described in more detail in WO 01/180761 and U.S. patent application Ser. No. 09/834,967.
In still another aspect, a bracket is provided with an improved bracket bonding pad that makes the brackets essentially self positioning, that is, it may be uniquely located and positioned on the teeth in the correct location with a positive fit without the use of a jig or other bracket placement mechanism, such as the tray as proposed by Cohen, U.S Pat. No. 3,738,005, or the jig of the Andreiko et al. patents. In particular, an improvement to a bracket having a bracket bonding pad is provided in which the bracket bonding pad has a tooth contacting surface of three-dimensional area extent conforming substantially exactly to the three-dimensional shape of the tooth where the pad is bonded to the tooth.
In one possible embodiment, the three-dimensional area extent is sufficiently large, and considerably larger than all bracket bonding pads proposed in the prior art, such that the bracket can be readily and uniquely placed by hand and located on the tooth in the correct location due to the substantial area extent corresponding to the three-dimensional surface of the tooth. The bracket is able to be bonded in place on the tooth without the assistance of a bracket placement aid such as a jig. In another possible embodiment, the area extent covers a cusp or a portion of a cusp to enable the bracket to uniquely placed on the tooth.
In another aspect, a bracket is provided with a bracket bonding pad that comprises a thin shell in order to reduce the overall thickness of the bracket as much as possible. The pad includes a tooth-facing surface conforming to the surface of the tooth. In this embodiment the bracket bonding pad has an opposite surface corresponding to the tooth-facing surface which has a three-dimensional surface configuration which also matches the three-dimensional surface of the tooth. In order to create a thin pad on a computer, a preferred method is to create a normal vector of each element of the bracket bonding pad""s tooth-facing surface (for instance, a triangle depending on how the surface is represented in the computer). Each surface element is xe2x80x9cshiftedxe2x80x9d in the direction of the normal vector away from the tooth using a pre-defined offset value corresponding to the thickness of the bonding pad. In this way, a thin shell is created, the outside of the shell having substantially the same area extent and three-dimensional surface corresponding to the tooth-facing surface of the bracket bonding pad. Other techniques could be used as well. For example, the bracket bonding pad could have a thinner periphery (e.g., 0.1 mm) and a thicker center portion (e.g., 0.3 mm) adjacent to where the bracket body is attached to the bonding pad. Appropriate software programs can be provided to vary the thickness over the surface of the bracket bonding pad, such as by scaling the normal vector with a variable depending on how close the normal vector is to the edge of the bracket bonding pad.
In yet another aspect of the invention, a method of designing a customized orthodontic bracket for a patient with the aid of a computer is provided. The bracket has a bracket bonding pad. The computer stores a three-dimensional model of the teeth of the patient. The method comprises the steps of determining an area of a tooth at which the bracket bonding pad is to be attached to the tooth; obtaining a three-dimensional shape of a tooth-facing surface of the bracket bonding pad, wherein the three-dimensional shape conforms to the three-dimensional shape of the tooth; and obtaining a three-dimensional shape of a second, opposite surface from the tooth-facing surface of the bracket bonding pad. A library of three-dimensional virtual bracket bodies is stored in the computer or otherwise accessed by the computer. The method continues with the step of obtaining a bracket body from the library and combining the bracket body with the bracket bonding pad to form one virtual three-dimensional object representing a bracket.
In a preferred embodiment, the second, opposite surface has a three-dimensional shape corresponding to the tooth-facing surface of said bracket bonding pad, for example, by performing the xe2x80x9cshiftingxe2x80x9d technique described earlier. The method may also incorporate the optional step of modifying the virtual model of the bracket body. For example, the bracket body may have a portion thereof removed in order to place the slot of the bracket body as close as possible to the bracket bonding pad and delete the portion of the bracket body that would otherwise project into the crown of the tooth. As another example, the modification may include adding auxiliary features to the bracket body such as hooks.
The addition of the bracket body to the bracket bonding pad with the aid of the computer may be performed for a group of teeth at the same time in order to take into account the proximity of adjacent teeth and brackets. Thus, the method may include the step of viewing, with the aid of the computer, a plurality of virtual teeth and virtual bracket bonding pads attached to the teeth, and shifting the location of the bracket body relative to its respective bracket bonding pad. This latter step would be performed for example in order to better position the bracket body on the bonding pad, or in order to avoid a conflict between the bracket body and an adjacent or opposing tooth such as a collision during chewing or during tooth movement.
In yet another aspect of the invention, a method is provided for designing and manufacturing a customized orthodontic bracket. The method includes the step of storing a digital representation of the relevant portion of the patient""s dentition in a computer. This could be a digital representation of either the entire dentition, or alternatively only the surfaces of the teeth upon which the brackets are to be bonded. The method continues with the steps of providing access to a library of virtual three-dimensional bracket bodies, such as for example storing the library in the computer, and determining the shape and configuration of bracket bonding pads, with the bracket bonding pads having a tooth-facing surface conforming substantially exactly to corresponding three-dimensional surfaces of the teeth. The method continues with the step of combining the bracket bodies from the library of bracket bodies with the bracket bonding pads to thereby create a set of individual, customized orthodontic brackets. A file representing the customized orthodontic brackets is exported from the computer to a manufacturing system for manufacturing the customized orthodontic brackets. The method continues with the step of manufacturing the customized orthodontic brackets, either using any of a variety of techniques known in the art such as milling, or one of the techniques described in detail herein such as casting.
Still other improvements are provided for manufacturing customized brackets. In one aspect, a method is provided of manufacturing an orthodontic bracket having a bracket body having a slot and a bracket bonding pad, comprising the steps of determining the three-dimensional shape of the orthodontic bracket and manufacturing the bracket from materials having at least two different hardnesses, a first relatively hard material or materials forming the bracket body and a second relatively soft material or materials forming the bracket bonding pad. The strength of the material of the bracket is always a compromise. While the section forming the slot should be as robust as possible to maintain the cross-section of the slot even when the bracket is exposed to high mechanical stress (e.g. by biting on hard objects), the section forming the pad should be softer to ease de-bonding after the treatment is finished. If the pad is soft enough, it can literally be peeled off the tooth surface, using an adequate tool. Depending on the type of the manufacturing process, it is possible to use different alloys to achieve such a configuration. Using centrifugal casting, first, a controlled amount of a hard alloy can be used to form the section that holds the slot, and afterwards a softer alloy is used to fill up the remainder of the bracket (or other way round). Controlling the amount of material needed to form a specific portion of the bracket is possible, since from the 3D models, the volume of each component of the bracket is precisely known. Other manufacturing techniques can be used, such as a laser sintering process, in which different alloy powders are used for the different layers.
In still another aspect, a modular approach to designing customized brackets for an individual patient is provided using a computer. The computer stores a library of virtual bracket bodies, virtual bracket bonding pads, and optionally virtual bracket auxiliary devices such as hooks. The user species or selects a bracket bonding pad and a bracket body for a particular tooth. The two virtual objects are united to form a virtual bracket. The user may be provided with graphics software tools to specify how and where the bracket body and bonding pad are united. Data representing the virtual bracket can be exported to a rapid prototyping process for direct manufacture of the bracket or manufacture of a template or model that is used in a casting process to manufacture the bracket. In one possible embodiment, the bracket bonding pad conforms substantially exactly to the surface of the tooth. Alternatively, the bracket bonding pad could be of a standard configuration.
These and still other principles of the various inventions set forth herein will be discussed in greater detail in conjunction with the appended drawings.